1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to a shingled magnetic recording (SMR) HDD that minimizes the effect of far track erasure (FTE) on data tracks in the boundary regions of data bands.
2. Description of the Related Art
Magnetic recording disk drives that use “shingle writing”, also called “shingled recording” or “shingled magnetic recording” (SMR), have been proposed, for example as described in U.S. Pat. No. 6,185,063 B1 and U.S. Pat. No. 6,967,810 B2. In SMR, the write head, which is wider than the read head in the cross-track direction, writes magnetic transitions by making a plurality of consecutive circular paths that partially overlap. The non-overlapped portions of adjacent paths form the shingled data tracks, which are thus narrower than the width of the write head. The data is read back by the narrower read head. The narrower shingled data tracks thus allow for increased data density. The shingled data tracks are arranged on the disk as annular bands separated by annular inter-band gaps or guard bands.
In application Ser. No. 13/135,953, filed Jul. 18, 2011, published as U.S. 2012/0300328 A1 and assigned to the same assignee as this application, a SMR disk drive is described that has on-disk cache. Writeable cache tracks are located in the inter-band gaps. Data from the host is transferred to the disk drive's internal memory, typically DRAM, and then written to cache tracks in an inter-band gap between two data bands. When the disk drive is idle, the data is then read from the cache tracks and written to a data band.
A problem in both conventional HDDs and SMR HDDs is wide-area track erasure (WATER) or far track encroachment or erasure (FTE). The write field from the write head is wider than a data track so when the write head is writing to a track, the outer portions of the write field (called the fringe field) overlap onto tracks other than the track being written. Data degradation due to fringe fields is not limited to the tracks immediately adjacent the track being written, but can extend over a range of tracks relatively far from the track being written. This FTE is particularly noticeable with write heads that have side shields. FTE may not affect tracks symmetrically on both sides of the track being written. Tracks on one side may encounter more pronounced FTE effects due to the write head shield design or due to read-write head skew. FTE is described by Liu et al., “Characterization of Skip or Far Track Erasure in a Side Shield Design”, IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009, pp. 3660-3663. U.S. application Ser. No. 12/831,391 filed Jul. 19, 2010, and assigned to the same assignee as this application, describes a conventional HDD where the effect of FTE is minimized by counting the number of writes, incrementing counters based on the known effect of FTE on each track within a range of the track being written, and then rewriting the data when a count reaches a predetermined threshold.
In a SMR disk drive, FTE can occur on the tracks in the boundary regions of bands, i.e., those tracks near the inter-band gaps, when data is written to the cache tracks in the inter-band gaps. What is needed is a SMR HDD that minimizes the effect of FTE from writing to the cache tracks.